Born in Valby, suburban Copenhagen, Claus Berthelsen was the youngest of four boys.
Inspired by a mother fond of singing and the father, an architect playing both piano and violin, Claus took a keen interest in buildings, song and music.
Singing in a boychoir he gained a sense of musical discipline, yet at the same time aquiring an immense distaste for authorities.

Coming out of high school he joined Henrik Balling and other ex-schoolmates in a band called Florence. Balling wrote the music, Berthelsen supplied the lyrics and sang as well.
Florence was shortlived, but became the basis of three bands that conquered the music scene in Denmark - more or less - in the eighties; Scatterbrain, Gangway and naïve.

The latter led by Berthelsen, using it as a vehicle for his popmusical idiosyncrasies.
The band began as a quartet under the subtle name Words and Welfare, but soon decided on naïve, just before Scatterbrain-guitarist Hilmer Hassig joined the group.
On the 1986-debut fish? naïve was a duo consisting of Berthelsen and Hassig, who subsequently recruited new musicians, taking the band on tour and eventually recording the second album Careless as a band in 1989.
That album was broadly acknowledged, drawing particular attention to the song Marble Afternoon and the titletrack.
After the release of a third album, Absolution Music in 1995, the collaboration between Berthelsen and Hassig dissolved, and although the band never officially broke up  - it silently ceased to exist.

Meanwhile, Berthelsen had grown accustomed to being a radiohost at the Danish National Radio - a job he is still deeply into.

During the years, following the demise of naïve, Berthelsen kept writing music, but didn't show any urge to engage in recording or publishing.
Not until late 2008, when a compilation of songs, dealing with the phenomenon of New Years, contained a song written by Berthelsen. Var det det du ønskede dig was his first song with lyrics in Danish - and, who knows, perhaps the last.

2008 also saw the tragic and untimely death of Berthelsens friend and collaborator Hilmer Hassig.
Being an outstanding guitar-player/producer - possibly the most important in danish music within the last three decades - Hassig was honoured with two memorial concerts in Copenhagen.
naïve reformed for the concerts to show hommage to Hilmer, but even though Berthelsen was in the process of writing new material, he was determined that the band as such should never appear again.
naïve was, in Berthelsens opinion, the work of Berthelsen/Hassig - the loss of Hilmer made it impossible to reconstruct.

So Berthelsen decided to let his new songs loose under a different name.
amazement - songs by Claus Berthelsen !

then still - after more consideration - Berthelsen decided to go back to working with naïve,
releasing a new album – entitled Endurance – January 2020.